What exactly was that bit of mischief Robert Mueller inserted in his final report, saying that "…while this report does not conclude that the President committed a crime, it also does not exonerate him?"

It's this simple: prosecutors are charged with finding crimes. If there is insufficient evidence to bring a case, then that is the end of the matter. Prosecutors, special or otherwise, are not authorized to offer hypothetical accounts where they can't bring a criminal case. But Mr. Mueller produced a brief of arguments pro-and-con about obstruction for others to decide upon. In doing that, he was out of order, and maliciously so.

Of course, Attorney General Barr took up the offer and declared the case closed, as he properly should where the prosecutor could not conclude that a crime was committed. One hopes that the AG also instructed Mr. Mueller and his staff to shut the fuck up vis-à-vis further ex post facto "anonymous source" speculation in the news media.